Power control for improved near-far performance of radar systems

ABSTRACT

A radar system includes a transmitter, a receiver and a processor. The receiver receives radio signal that includes the transmitted radio signal reflected from targets in the environment. The received radio signal is provided to the processor. The processor samples the received radio signal during a plurality of time intervals to produce a sampled stream. The different time intervals of the plurality of time intervals will contain different signal levels of radio signals reflected from the targets. The processor also selects a particular time interval of the plurality of time intervals that is free of samples of radio signals reflected off of the targets that are closer than a first threshold distance from an equipped vehicle. The transmitter and/or the receiver is adjusted such that samples of radio signals reflected off of targets that are farther than the first threshold distance are detected.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 15/690,899, filed Aug. 30, 2017, now U.S. Pat. No. 9,829,567,which is a continuation of U.S. patent application Ser. No. 15/292,755,filed Oct. 13, 2016, now U.S. Pat. No. 9,753,121, which claims thefiling benefits of U.S. provisional applications, Ser. No. 62/371,257,filed Aug. 5, 2016, Ser. No. 62/361,587, filed Jul. 13, 2016, and Ser.No. 62/352,277, filed Jun. 20, 2016, which are all hereby incorporatedby reference herein in their entireties.

FIELD OF THE INVENTION

The present invention is directed to radar systems, and in particular toradar systems for vehicles.

BACKGROUND OF THE INVENTION

The use of radar to determine range and velocity of objects in anenvironment is important in a number of applications includingautomotive radar and gesture detection. A radar typically transmits asignal and listens for the reflection of the signal from objects in theenvironment.

SUMMARY OF THE INVENTION

The present invention provides methods and a system for achieving betterperformance in a radar system when there is a near object and a farobject. The invention accomplishes better detectability of the farobject in the presence of a near object by altering the transmissionpower as a function of time.

A radar sensing system for a vehicle comprises at least one transmitter,at least one receiver, and a processor. The at least one transmitter isconfigured for installation and use on a vehicle, and able to transmit aradio frequency (RF) signal. The at least one transmitter is furtherable to power shape the transmitted RF signal. The transmitted RF signaldecreases in power over a plurality of time intervals. The at least onereceiver is configured for installation and use on the vehicle, and ableto receive a reflected RF signal. The reflected RF signal is thetransmitted RF signal reflected from targets in the environment. Thereflected RF signal is down-converted and the result provided to theprocessor. The processor samples the down-converted reflected RF signalto produce a sampled stream. The processor is able to process thesampled stream during the plurality of time intervals. The differenttime intervals of the plurality of time intervals will contain differentsignal levels of RF signals reflected from near and far targets. Theprocessor is further able to select samples in the sampled stream over aselected time interval of the plurality of time intervals that is freeof RF signals reflected off of the near targets.

A radar sensing system for a vehicle comprises at least one transmitter,at least one receiver, and a processor. The at least one transmitter isconfigured for installation and use on a vehicle, and able to transmit aradio frequency (RF) signal. The at least one transmitter is furtherable to transmit the RF signal on a selected frequency during a selectedtime interval of a plurality of time intervals and to transmit the RFsignal on another selected frequency during another selected timeinterval of the plurality of time intervals. The at least one receiveris configured for installation and use on the vehicle, and able toreceive a reflected RF signal. The reflected RF signal is thetransmitted RF signal reflected from objects in the environment. Thereflected RF signal is down-converted and the result provided to theprocessor. The processor samples the down-converted reflected RF signalto produce a sampled stream. The processor is able to process thesampled stream during the plurality of time intervals. The differenttime intervals of the plurality of time intervals will contain differentsignal levels of RF signals reflected from targets. The processor isfurther operable to select samples in the sampled stream from a selectedtime interval of the plurality of time intervals that is free of RFsignals reflected off of near targets.

A radar sensing system for a vehicle comprises at least one transmitter,at least one receiver, and a processor. The at least one transmitter isconfigured for installation and use on a vehicle, and able to transmit aradio frequency (RF) signal. The at least one transmitter is furtherable to power shape the transmitted RF signal. The transmitted RF signaldecreases in power over time to compensate for the mismatchedattenuations between near and far targets. The at least one transmitteris further able to transmit the RF signal on a selected frequency of aplurality of frequencies during a selected time interval of a pluralityof time intervals and to transmit the RF signal on another selectedfrequency during another selected time interval of the plurality of timeintervals. The at least one receiver is configured for installation anduse on the vehicle, and able to receive a reflected RF signal on aplurality of frequencies. The reflected RF signal is the transmitted RFsignal reflected from targets in the environment. The reflected RFsignal is down-converted and the result provided to the processor. Theprocessor samples the down-converted reflected RF signal to produce asampled stream. The processor is able to process the sampled streamduring a plurality of time intervals. The different time intervals ofthe plurality of time intervals will contain different signal levels ofRF signals reflected from the targets. The processor is further able toselect samples in the sampled stream over a selected time interval ofthe plurality of time intervals that is free of RF signals reflected offof near targets.

Signal processing of RF signals reflecting from near targets may occurduring a first time interval of the plurality of time intervals, whilesignal processing of RF signals reflecting from distant targets mayoccur during a second time interval of the plurality of time intervalsthat is after the first time interval. During the first time interval,the transmitted RF signal may be decreasing in power, and during thesecond time interval, there is no transmitted RF signal. During thefirst time interval, the at least one receiver may ignore the reflectedRF signal, and during the second time interval, the at least onereceiver may receive the reflected RF signal. The reflected RF signalhas a return signal strength of 1/r4, such that near and distant targetshave a similar RCS.

The variable power is such that the RF signal reflected from a nearobject may have a lower transmitted power than the RF signal reflectedfrom a most distant object at a given time. That is, a large amplitudeRF signal is initially transmitted. The transmitted power is thendecreased as a function of time. The processor may be further able toselect a time interval of the plurality of time intervals to process aselected plurality of range bins. The RF signals reflected from thefirst near target are not large enough to interfere with RF signalsreflected from the second distant target.

These and other objects, advantages, purposes and features of thepresent invention will become apparent upon review of the followingspecification in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an automobile equipped with a radar system inaccordance with the present invention;

FIG. 2A and FIG. 2B are block diagrams of radar systems in accordancewith the present invention;

FIG. 3 is an block diagram illustrating a flow of information through aradar receiver of the radar system of FIGS. 2A and 2B;

FIG. 4 is a block diagram illustrating a radar system with a pluralityof receivers and a plurality of transmitters in accordance with thepresent invention;

FIG. 5 is a two-dimensional array illustrating the time slice of FIG. 3in accordance with the present invention;

FIG. 6 is a block diagram illustrating another radar system with aplurality of receivers and a plurality of transmitters in accordancewith the present invention;

FIG. 7 is a block diagram of an exemplary system on a chip architecturefor the radar system of the present invention;

FIGS. 8-10 illustrate aspects of how digital radar works;

FIG. 11 illustrates capabilities of a multiple transmitter and multiplereceiver radar system;

FIG. 12 is a block diagram of an exemplary receive pipeline and transmitpipeline for the radar system of the present invention;

FIG. 13 is a pair of graphs illustrating the output of correlators withdelays of the replica for two objects with continuous transmission;

FIG. 14 is a graph illustrating the correlator outputs for total signalstrength from the sum of the outputs from FIG. 13;

FIG. 15 is a pair of graphs illustrating the output of correlators withdelays of the replica for two objects with discontinuous transmission inaccordance with the present invention;

FIG. 16 is a graph illustrating the correlator outputs for total signalstrength from the sum of the outputs from FIG. 15 in accordance with thepresent invention;

FIG. 17 is a graph illustrating a portion of the total signal strength(zoomed in) of FIG. 16 in accordance with the present invention;

FIG. 18 is a flow diagram illustrating the effect of a discontinuoustransmission that is reflected from two objects at different distancesin accordance with the present invention;

FIG. 19 is a flow diagram illustrating the timing of the processing ofthe reflected signals of FIG. 18 in accordance with the presentinvention;

FIG. 20 is a flow diagram illustrating the timing of the processing ofthe reflected signals of FIG. 18 in accordance with the presentinvention;

FIG. 21 is a flow diagram illustrating a power shaping of thetransmission, a timing of processing of reflected signals, and acorresponding plot of reflected signal strength in accordance with thepresent invention;

FIG. 22 is a flow diagram further illustrating the timing of processingof reflected signals and a corresponding plot of reflected signalstrength in accordance with the present invention;

FIG. 23 is a block diagram of an exemplary receiver for the radar systemof the present invention;

FIG. 24 is an exemplary timing diagram for objects at differentdistances in accordance with the present invention;

FIG. 25 is a block diagram of an exemplary digital processor fordetermining signal correlation with required spreading codes inaccordance with the present invention;

FIG. 26 illustrates FFT inputs for the receiver of FIG. 25 in accordancewith the present invention;

FIG. 27 is a graph illustrating normalized correlator outputs forobjects at 150 and 300 meters when signal processing is performed from2.0-3.832 microseconds;

FIG. 28 is a graph illustrating normalized correlator outputs forobjects at 150 and 300 meters when signal processing is performed from2.5-3.832 microseconds; and

FIG. 29 is a graph illustrating normalized correlator outputs forobjects at 150 and 300 meters when signal processing is performed from2.832-3.832 microseconds.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described with reference to theaccompanying figures, wherein numbered elements in the following writtendescription correspond to like-numbered elements in the figures. Methodsand systems of the present invention may achieve better performance froma radar system when there is a near object and a far object. Exemplaryembodiments of the present invention accomplishes better detectably ofthe far object in the presence of a near object by altering thetransmission power of the radar system as a function of time.

The present invention provides methods and a system for achieving betterperformance in a radar system when there is a near object and a farobject. The invention accomplishes better detectability of the farobject in the presence of a near object by altering transmission poweras a function of time.

A radar system utilizes one or more transmitters to transmit radiofrequency (RF) signals. These RF signals are reflected from objects inthe environment, also called targets, by one or more receivers of theradar system. The radar system uses one or more receivers to receive RFsignals. A transmitter/receiver pair is called a virtual radar. Thereceivers of the radar system have a down-converter that outputs complexvalued digitized samples (i.e., values comprising a mathematical realcomponent and a mathematical imaginary component) to a processor. Thecomplex-valued digitized samples are correlated with varioustime-delayed replicas of the transmitted RF signals for differentreceivers. These samples are arranged in two-dimensional arrays known astime slices. The samples are placed into respective range bins of thetwo-dimensional array (as used herein, a range bin refers to a distancerange corresponding to a particular time delay corresponding to theround trip time of the radar signal from a transmitter, to thetarget/object, and back to the receiver). The virtual receivers of theradar system (a virtual receiver for each virtual radar of the radarsystem) define an x-axis of the two-dimensional time slice and the rangebins define a y-axis of the two-dimensional time slice. Another timeslice comprising complex-valued samples is generated every 2-30microseconds. Over a longer time interval, herein referred to as a“scan” (typically, in a duration of 1-60 milliseconds or longer),multiple time slices are accumulated to form a three-dimensional radardata cube. An x-axis of the three-dimensional radar data cube is definedby time (of each respective time slice requiring 2-30 microseconds),while the receivers (or virtual receivers) define a z-axis of thethree-dimensional radar data cube, and the range bins and theircorresponding time delays define a y-axis of the three-dimensional radardata cube. A radar data cube may have a preselected or dynamicallydefined quantity of time slices. For example, a radar data cube mayinclude a 100 time slices or a 1000 time slices of data. Similarly, aradar data cube may include different numbers of range bins.

The complex-valued samples of the received RF signals are the productsof a correlation between multiple time-delayed replicas of thetransmitted RF signals—with a time-delayed replica corresponding to eachrange bin—and the received RF signals. When a particular time-delayedreplica in a particular range bin correlates highly with the received RFsignal, it is an indication of the time delay (i.e., range of thetarget/object) for the transmitted RF signal that is received afterreflecting from a target/object. As discussed herein, each time slicecontains one sample in a time series of samples upon which Dopplerprocessing is performed (e.g., a Fast Fourier Transform). In otherwords, a time series of samples comprises the samples of a particularrange bin for a particular receiver (or virtual receiver/radar). Thelonger the chain of time series of samples, the higher the Dopplerresolution.

The transmitted radio frequency (RF) signals are reflected from objectsin the environment and are received back at the radar receivers (orvirtual receivers/radars). The transmitted signal from each radartransmitter consists of a baseband signal which is upconverted to an RFsignal by an RF upconverter followed by an antenna. The received signalat each radar receiver antenna is downconverted by an RF downconverterto a complex baseband signal. The baseband transmitted RF signals andthe reflected RF signals after down-conversion in the receiver areprovided to the processor. As an example, a baseband signal used fortransmission by one transmitter of the radar system might consist ofrepeated sequences of random or pseudo-random binary values for onetransmitter, e.g., (−1, −1, −1, −1, 1, 1, 1, −1, 1, 1, −1, −1, 1, −1,1), although any sequence, including non-binary sequences andnon-periodic sequences, could be used and different sequences would beused for different transmitters.

The transmitted signal at a radio frequency may be obtained by mixing abaseband signal with a local oscillator signal. At the receiver (receivepipeline), the received RF signal is down-converted by typicalamplification, filtering, and mixing with in-phase and quadrature-phasecomponents of an oscillator. The resulting complex signal afterdown-conversion is sampled and then correlated with different delays(time shifts) of replicas of the baseband signal to produce acomplex-correlation value for each period of the binary sequence. Aparticular correlator that has a replica that is matched in delay to thetime delay of the reflected RF signal from an object would produce alarge magnitude complex correlator output. A single correlator willproduce a sequence of correlator outputs that will be large if thereflected signal has a delay that matches the delay of the replica ofthe baseband transmitted signal. If there is a different velocity of theradar system as compared to an object causing a reflection, there willbe a Doppler shift in the frequency of the reflected RF signal relativeto the transmitted RF signal. A sequence of correlator outputs for oneparticular delay will have complex values that rotate at a rate relatedto the Doppler shift. Using a sequence of correlator outputs (alsoreferred to as a scan), the Doppler shift may be estimated and thus thevelocity of the object in the environment can be determined. The longerthe sequence of correlator outputs used to estimate the Dopplerfrequency, the greater the accuracy and resolution of the estimation ofthe Doppler frequency, and thus, the greater the accuracy in estimatingthe velocity of the object.

There may be scans for different correlators that use replicas of thetransmitted signal with different delays. Because there are multipletransmitters and multiple receivers, there may be correlators thatprocess a received RF signal at each receiver that are matched to aparticular transmitted RF signal by a particular transmitter. Eachtransmitter/receiver pair is called a “virtual radar” (a radar systempreferably has 4 virtual radars, or more preferably 32 virtual radars,and most preferably 256 or more virtual radars). The receive pipeline ofeach receiver of the radar system will thus generate a sequence ofcorrelator outputs (time slices) for each possible delay and for eachvirtual receiver/radar. This set of data is called a radar data cube(RDC). The delays are also called range bins. The part of the radar datacube for one point in the sequence of correlator outputs is called atime slice, and it contains one correlator output for each range bin andvirtual receiver/radar combination. Storing the radar data cube caninvolve large amount of memory, as its size depends on the desirednumber of virtual radars (for example, 4-64 or more virtual radars), thedesired number of range bins (for example, 100-500 or more range bins),and the desired number of time slices (for example, 200-3000 or moretime slices).

The complex-valued digitized samples contained in a three-dimensionalradar data cube may be processed, preferably by a processor establishedas a CMOS processor and coprocessor on a common/same semiconductorsubstrate, typically a silicon substrate. In one embodiment, theprocessor comprises fixed function and programmable CPUs and/orprogrammable logic controls (PLCs). Preferably, the system will beestablished with a radar system architecture (including, for example,analog RF circuitry for the radar, processor(s) for radar processing,memory module(s), and other associated components of the radar system)all on a common/same semiconductor substrate. The system may preferablyincorporate additional processing capabilities (such as, for example,image processing of image data captured by one or more vehicle camerassuch as by utilizing aspects of the systems described in U.S. Pat. Nos.5,877,897; 5,796,094; 6,396,397; 6,690,268 and 5,550,677, which arehereby incorporated herein by reference in their entireties) within thecommon/same semiconductor substrate as well.

The ability of a continuous-wave radar system to distinguish multipleobjects/targets is dependent upon the radar system's range, angle, andDoppler resolutions. Range resolution is limited by a radar's bandwidth(i.e., the chip rate in a phase modulated continuous wave radar), whileangle resolution is limited by the size of the antenna array aperture.Meanwhile, increasing Doppler resolution only requires a longer scan. Ahigh Doppler resolution is very valuable because no matter how close twoobjects or targets are to each other, as long as they have slightlydiffering radial velocity (their velocity towards or away from the radarsystem), they can be distinguished by a radar system with a high enoughDoppler resolution. Consider a walking adult next to a walking child,where the adult is moving towards the radar system at 1.5 meters persecond while the child is moving towards the radar system at 1.2 metersper second (ignoring how fast the radar system may be moving). If theDoppler resolution of the radar system is high enough, the radar systemwill be able to distinguish the two targets. However, if the radarsystem is only able to achieve Doppler resolutions of up to an exemplary0.5 meters per second, the radar system will be unable to distinguishthe two targets. Preferably, the Doppler resolution is 0.1 meter persecond (m/s) and more preferably less than 0.05 m/s.

FIG. 1 illustrates an exemplary radar system 100 configured for use in avehicle 150. In an aspect of the present invention, a vehicle 150 may bean automobile, truck, or bus, etc. As illustrated in FIG. 1, the radarsystem 100 may comprise one or more transmitters and one or morereceivers 104 a-104 d for a plurality of virtual radars, control andprocessing module 102 and indicator 106. Other configurations are alsopossible. FIG. 1 illustrates receivers/transmitters 104 a-104 d placedto acquire and provide data for object detection and adaptive cruisecontrol. The radar system 100 (providing such object detection andadaptive cruise control or the like) may be part of an Advanced DriverAssistance System (ADAS) for the automobile 150.

FIG. 2A illustrates an exemplary radar system 200 with an antenna 202that is time-shared between a transmitter 206 and a receiver 208 via aduplexer 204. As also illustrated in FIG. 2A, output from the receiver208 is received by a control and processing module 210 that processesthe output from the receiver 208 to produce display data for the display212. As discussed herein, the control and processing module 210 is alsooperable to produce a radar data output that is provided to othercontrol units. The control and processing module 210 is also operable tocontrol the transmitter 206. FIG. 2B illustrates an alternativeexemplary radar system 250 with a pair of antennas 202 a, 202 b, aseparate antenna 202 a for the transmitter 206 and another antenna 202 bfor the receiver 208. While pulse radar systems may use shared orseparate antennas, continuous-wave radars (discussed herein) will useseparate antennas (for transmitting and receiving) because of theircontinuous operation.

The radar sensing system of the present invention may utilize aspects ofthe radar systems described in U.S. patent application Ser. No.15/204,003, filed Jul. 7, 2016, now U.S. Pat. No. 9,575,160, and/or Ser.No. 15/204,002, filed Jul. 7, 2016, now U.S. Pat. No. 9,599,702, and/orU.S. provisional applications, Ser. No. 62/319,613, filed Apr. 7, 2016,Ser. No. 62/327,003, filed Apr. 25, 2016, Ser. No. 62/327,004, filedApr. 25, 2016, Ser. No. 62/327,005, filed Apr. 25, 2016, Ser. No.62/327,006, filed Apr. 25, 2016, Ser. No. 62/327,015, filed Apr. 25,2016, Ser. No. 62/327,016, filed Apr. 25, 2016, Ser. No. 62/327,017,filed Apr. 25, 2016, and/or Ser. No. 62/327,018, filed Apr. 25, 2016,which are all hereby incorporated by reference herein in theirentireties.

There are several types of radio frequency (RF) signals used in radarsystems. One type of radar RF signal is known as a frequency-modulatedcontinuous-waveform (FMCW). In an FMCW radar system, the transmitter ofthe radar system sends a continuous signal in which the frequency of theRF signal varies. This is sometimes called a chirp radar system. At thereceiver a matched filter may be used to process the received RF signal.The output of the matched filter is a so-called “pulse-compressed”signal with a pulse duration inversely proportional to the bandwidthused in the chirp signal.

Another type of radar RF signal is known as a phase-modulatedcontinuous-waveform (PMCW). For this type of RF signal, the phase of thetransmitted RF signal is changed according to a certain pattern or code,sometimes called a spreading code, which is known at the radar receiver(see FIGS. 8-10). The faster the phase is changed, the wider thebandwidth of the transmitted RF signal. This is sometimes calledspread-spectrum because the signal power is spread over a widebandwidth. As illustrated in FIGS. 8-10, the pattern may be implementedas a pseudorandom binary code, where 1s and 0s indicate a positive ornegative phase shift. As also illustrated in FIGS. 9 and 10, a phaseshift does not always affect the waveform (e.g., a binary code sequenceof 1, 1 or 0, 0 does not change the phase of the waveform for the second1 or 0). In an aspect of the present invention, there may be differenttypes of codes, with each transmitter of a radar system using a separatecode. In other modes, each transmitter transmits with a single code thatis modulated according to a Hadamard transform.

At the receiver, a matched filter is used that produces a so-calledpulse compressed signal with time resolution proportional to the inversebandwidth of the transmitted RF signal. Codes with good autocorrelationvalues are important in single transmitter, single receiver,phase-modulated continuous-wave radars.

Radars with a single transmitter and a single receiver can determinedistance to a target but cannot determine a direction or an angle of atarget from the radar sensor or system. To achieve angular information,either multiple transmitters or multiple receivers or both are needed.The larger the number of transmitters and receivers, the better theresolution possible. A system with multiple transmitters and multiplereceivers is also called a multiple-input, multiple-output or MIMOsystem. With MIMO radar systems, the receive pipeline for each receiverof the radar system can have a matched filter for each of thetransmitter codes. As discussed herein, a quantity of virtual radars isdefined by the quantity of transmitters and physical receivers (aquantity of transmitters times a quantity of physical receivers equals aquantity of virtual radars). A receiver may be referred to as a virtualreceiver. A radar system with only a single transmitter will havevirtual receivers that are physical receivers, but may still be referredto as virtual receivers.

One type of radar system transmits the same type of signal continuously.The signal might be a frequency-modulated continuous-wave (FMCW) type ofwaveform that sweeps a continuous-wave (CW) signal in frequency acrosssome range of frequencies. Mixing (multiplying) the reflected wave froma target with a replica of the transmitted signal results in a CW signalwith a frequency that represents the distance between the radartransmitter/receiver and the target. By sweeping up in frequency andthen down in frequency the Doppler frequency can also be determined.Phase-modulated continuous-wave (PMCW) radar systems on the other handtransmit an RF signal in which a phase is modulated (e.g., with twovalues 0 and −π). For a single transmitter, a sequence of phasevalues—sometimes called a spreading code—that has good autocorrelationproperties is required so that ghost targets are minimized. The rate atwhich the phase is modulated determines the bandwidth of the transmittedRF signal.

An exemplary radar system determines the range of an object or thedistance to an object by determining how long it takes an echo of atransmitted RF signal to be heard back at the receive pipeline of theradar system. From this measured time delay and knowing that theelectromagnetic RF signals travel at the speed of light (or ultrasonicsignals traveling at the speed of sound), the distance can bedetermined. A typical way of determining the time delay is bycorrelating a received RF signal with multiple time-delayed replicas ofthe transmitted RF signal (via the use of range bins, as discussedherein). When a particular time-delayed replica corresponding to aparticular range bin correlates highly with the received RF signal(which results in a larger amplitude signal as compared to a RF signalthat does not correlate highly with the received RF signal), it is anindication of the time delay (or equivalent range) for the transmittedRF signal that is received after reflecting from an object. Becausethere can be multiple objects in the environment, there will be multipletime delays for which there will be a high correlation. While a virtualreceiver/radar could correlate the received RF signal with all possibledelays, generally there is a finite set of delays with which the virtualreceiver/radar will correlate (that is, the range bins).

The ranges corresponding to the different time delays generated byreplica signals are the above mentioned range bins. They may also beknown as “range gates.” The accuracy of range determination increases asthe bandwidth of the transmitted RF signal increases (as the rate atwhich the phase of the signal changes) and as the number of time-delayreplicas increase. A receiver that uses a matched filter may generatecorrelation values for all possible time delays (each range bin). Thismatched filter receiver will still have a range resolution that dependson the bandwidth of the transmitted signal.

A radar system can determine the velocity of an object by measuring thefrequency offset between the transmitted RF signal and the received RFsignal. The RF signal reflecting from a moving object will have afrequency shift relative to the transmitted RF signal. This is calledthe Doppler effect and the frequency shift is called the Doppler shift.A noticeable exemplary Doppler effect may be experienced with regards totrains passing through a road crossing where the frequency of the soundwill go down once the train has passed the road crossing. The RF signalreflected off an object moving toward the radar system will have ahigher frequency than the transmitted RF signal, and an object movingaway will have a lower frequency. If the radar system is also moving,then it is the relative velocity between the radar system and the objectthat will determine the amount of frequency shift. Thus, measuring thefrequency shift will determine the relative velocity of an object in theenvironment. The time duration of the transmitted RF signal willdetermine the accuracy or resolution of the Doppler frequency.

The correlation of replicas of the transmitted RF signal modified atdifferent time delays and different frequency shifts is called theambiguity function. It is the correlation at different delays anddifferent frequency shifts which are an indication of the range andvelocity of the objects in the environment. A virtual radar calculatingthe ambiguity function can estimate the range and velocity of objects inthe environment.

Because there can be multiple objects in the environment with differentranges and different velocities, there may be multiple reflections withdifferent delays and different frequency shifts. It is important in manyapplications to measure the frequency shift for different objects atdifferent ranges to determine the velocity of individual objects. Eachobject in a given range will generate a frequency shift that should beestimated.

The frequency shift of a received signal at a particular range bin canbe measured by looking at the sequence of phase values of the correlatoroutput of the receive pipeline over a set of correlations. The phaseshift Δφ between two consecutive correlator outputs that are separatedin time ΔT will correspond to a frequency shift, Δf, that can bedetermined as Δf=Δφ/ΔT. The longer the time duration of the transmittedsignal, the more accurate the determination of frequency shift of anobject at a particular range bin.

While the description above is focused on a single transmitter-receiverpair, if there are N transmitters (TxN) and N receivers (RxN), therewill be TxN×RxN virtual radars, one for each transmitter-receiver pair.For example, a radar system with eight transmitters and eight receiverswill have 64 pairs or 64 virtual radars (with 64 virtual receivers). Asillustrated in FIG. 11, when three transmitters (Tx1, Tx2, Tx3) arebeing received by three receivers (Rx1, Rx2, Rx3), each of the receiversis receiving the transmission from each of the transmitters. Each of thereceivers is receiving the sum of all three of the transmissions at thesame time. Each receiver can attempt to determine the range and Dopplerof objects by correlating with delayed replicas of the signal from oneof the transmitters. The physical receivers may then be “divided” intothree separate virtual receivers, each virtual receiver correlating witha replica of one of the transmitted signals. In a preferred radar systemof the present invention, there are 1-4 transmitters and 4-8 receivers,or more preferably 4-8 transmitters and 8-16 receivers, and mostpreferably 16 or more transmitters and 16-64 or more receivers.

Collecting and storing all the information to determine the range andvelocity of each object corresponding to each virtual receiver requiressignificant memory resources. The required memory resources necessary toachieve sufficient Doppler resolution may run into the tens of megabytesto a 100 megabytes or more. For example, the amount of memory necessaryto store the data for a single scan of 1000 time slices from 64 virtualradars when that scan accumulates samples across 100 range bins for the64 virtual radars would be more than 25 megabytes (see FIG. 3). Inaccordance with an aspect of the present invention, there are preferably100 range bins, more preferably 200 range bins, and most preferably 500range bins in each time slice.

FIG. 4 illustrates an exemplary phase-modulated continuous-wave radarsystem 400. As illustrated in FIG. 4, the radar system 400 comprises aplurality of receivers and their respective antennas 406 and a pluralityof transmitters and their respective antennas 408. The radar system 400also includes a flash memory 412, and optionally a random access memory410. The random access memory 410, for example, an external DRAM, may beused to store radar data cube(s) instead of using the limited internal(on-chip) memory (e.g., SRAM), and may also be used to store selectedrange bins from a greater number of radar data cubes for concatenatingfor micro-Doppler processing. The radar system also includes a varietyof interconnections to an automotive network, e.g., Ethernet, CAN-FD,and Flexray.

Radar systems have the ability to measure a target's range and angle(azimuth and elevation) and Doppler velocity (range rate of change). Theprimary metrics in each of these measurements are resolution andaccuracy. Resolution is particularly important because it allows theradar system to distinguish (resolve) targets that are closely spaced inrange, angle, or Doppler velocity. Range resolution is limited by theradar's bandwidth—chip rate in a PMCW radar. Angle resolution is limitedby the size of the antenna array aperture. It is quite common that twoneighboring targets cannot be resolved in range or in angle due tophysical limitations of the radar. Increased Doppler resolution, on theother hand, only requires a longer scan. No matter how close two targetsare to each other, as long as they have slightly differing radialvelocity, they can be distinguished by a radar with a sufficiently highenough Doppler resolution. Two examples are (1) a pedestrian walking outinto the roadway from between two cars parked along the side of theroad, and (2) a motorcycle driving alongside a truck. The radar systemof the present invention provides greater sensitivity (to enable thedetection of pedestrians or wooden poles for parking) and enhancedresolution in range, azimuth, and elevation. The radar system providesenhanced target discrimination, while providing a cost effective devicethat provides scalability.

The PMCW radar system provides fully digital modulation, with featuresbecoming all digital vs analog. The RF is simpler and smaller, withabout a 10× advantage in size over FMCW systems. The radar systemprovides more area for digital processing, and provides improved rangeresolution, enhanced range isolation—very small targets next to largetargets, enhanced interference mitigation architecture (it can handle anorder of magnitude more interferers than FMCW), and enhanced isolationfrom interfering radars. The radar system provides true MIMO processing,which enables unmatched angular and super resolution. The PMCW radarsystem is also naturally resistant to jamming. The above described PMCWradar system when comprising multiple transmitters and multiplereceivers in a multiple-input-multiple-output (MIMO) system is useful ina radar system where the receivers and transmitters need to receive andtransmit simultaneously.

FIG. 6 illustrates another exemplary radar system of the presentinvention. Similar to the radar system 400 of FIG. 4, the radar systemof FIG. 6 comprises a plurality of transmitter and receiver antennas.The radar system of FIG. 6 also includes RAM and flash memories, as wellas interconnections to the automobile. FIG. 7 illustrates an exemplarysystem on chip architecture for the radar system of FIG. 6.

FIG. 12 illustrates an exemplary receive pipeline 1210 and an exemplarytransmit pipeline 1202 for a radar system of the present invention. Asillustrated in FIG. 12, a code generator outputs a code that is used toform a baseband signal that is upconverted to an RF signal by an RFupconverter followed by a transmission antenna. As illustrated in FIG.12, the received RF signal at a receiver antenna is down-converted bythe receive pipeline 1210's amplifiers, filters, and mixers andconverted into a complex baseband signal. The resulting signal afterdown-conversion is sampled by an analog-to-digital converter (ADC) andcorrelated by a correlator of the receive pipeline 1210 with differentdelays (time shifts) of replicas of the transmitted sequence to producea complex correlation value for each period of the binary sequence. Aparticular correlator that has a replica that is matched in delay to thetime delay of the reflected RF signal from an object would produce alarge magnitude complex correlator output. A single correlator willproduce a sequence of correlator outputs that will be large if thereflected RF signal has a delay that matches the delay of the replica ofthe baseband transmitted signal. If there is a difference between avelocity of the radar system and a velocity of the object causing thereflection, there will be a Doppler shift in the frequency of thereflected RF signal relative to the transmitted RF signal. A sequence ofcorrelator outputs for one particular delay will have complex valuesthat rotate at a rate related to the Doppler shift. Using a sequence ofcorrelator outputs (also referred to as a scan), the Doppler shift maybe estimated and thus the velocity of the object in the environment. Thelonger the sequence of correlator outputs used to estimate the Dopplerfrequency, the greater the accuracy and resolution of the estimation ofthe Doppler frequency, and thus the velocity of the object.

There may be scans for different correlators that use replicas of thetransmitted RF signal with different delays. Because there are multipletransmitters and multiple receivers (virtual radars), there may becorrelators that process a received RF signal at each receiver that arematched to a particular transmitted signal by a particular transmitter.The receive pipeline of 1210 for each receiver of the radar system willthus generate a sequence of correlator outputs for each possible delayand for each virtual radar. This set of data is called a radar data cube(RDC) made up of individual time slices.

In accordance with an aspect of the present invention, the receivepipeline 1210 comprises a correlator block, where the correlator blockcomprises a bank of correlators with a correlator for every range binand every virtual radar combination. In other words, a two-dimensionalbank of correlators similar to the two-dimensional array of range binsillustrated in FIG. 5. Optionally, the bank of correlators may besmaller than the array of range bins. For example, there may be onecorrelator for each physical receiver, such that each correlatorgenerates an output for each virtual receiver of the correspondingphysical receiver. Optionally, the entire receive pipeline 1210 afterthe receiver antenna(s) may be implemented on a single integrated chip.Optionally, even the antenna may be implemented on the same singleintegrated chip as the rest of the receive pipeline 1210. In accordancewith another aspect of the present invention, the correlatorfunctionality may be implemented in a receive pipeline. Optionally, thecorrelator functionality may be implemented in a processor. Optionally,portions of the receive pipeline may be implemented in a processor.

Pulsed Pmcw Radars for Improved Object/Target Detection:

Phase-modulated continuous-wave (PMCW) radars using pseudorandom binarysequence (PRBS) codes, almost perfect autocorrelation sequences (APAS),Golay codes, as well as many other codes, all have range sidelobes thatlimit the dynamic range of the radar. For PMCW radar systems thatutilize PRBS codes, the range sidelobes can be shown to be10*log(Ts*Rc*Vr) dB smaller than the peak correlation, or 10*log(totalchips*Vr) dB, where Vr is the number of virtual receivers, Ts is thescan time, Rc is the chip rate, and the product represents the number ofchips processed in the correlator. For a typical radar system with ascan time of 10 ms, a 1 GHz chip rate, and 16 virtual radars (Vr), thisrepresents a sidelobe level of 10*log(160e6) or 82 dB. Many applicationsfor automotive radar require 90-105 dB of dynamic range between thelargest to smallest targets. As an example, consider an automobile, suchas a truck at a range of 10 meters with a 40 dBsm RCS and a motorcycleat a range of 250 meters with a 0 dBsm RCS. The difference in the signalstrength from these objects assuming a free space path loss (in eachdirection) results in the truck having a stronger signal than themotorcycle (e.g., 40*log(250/10)+(40-0)=95 dB). This implies that thesidelobes due to the truck at 10 meters would make it difficult todetect the motorcycle at 250 meters. Another consideration is that thelargest target typically detected is often self-interference from abumper or antenna coupling. This could add another 60 dB of dynamicrange requirement if not handled properly.

Exemplary methods may be used to cancel out the signal or reduce thesidelobes of the targets, but all of these rely on an accuratemeasurement of: the distance to the target with sub-range bin accuracy,the phase of the target to within less than 1 degree, and the amplitudeof the target. Unfortunately, all of these values are related to the SNRof the target and might not be able to cancel out properly withouthighly accurate measurements of these three parameters. In addition, thetransfer function of the channel must be accurately known or determinedto remove the signal or to remove the sidelobes directly. The channeltransfer function is changing with the temperature of the sensing device(among other factors), making it difficult to estimate the “what” and“how much” to cancel out in order to remove the signal from the nearobject. Beyond the channel transfer function, there are alsonon-linearities that need consideration. These include second and thirdharmonics, phase pulling of the transmitted or received RF signals,mixer issues, spectral emission mask signal reduction, and a sundry ofother non-linearities that are common in analog blocks that can be 20 to30 dBc down. Some of these require sampling at significantly higherfrequencies so that they can be resolved.

Exemplary embodiments of the present invention provide methods and asystem for achieving better performance in a radar system when there isa near object and a far target. Better detectability is accomplished forthe far target in the presence of a near target by altering thetransmission power as a function of time. Such embodiments providecompensation for the mismatched attenuations between the near and fartargets (without compensation, the signals reflected off of near targetshave a higher signal strength than the signals reflected off of fartargets).

In an aspect of the present invention, an exemplary transmittertransmits a radio frequency (RF) signal with variable transmission poweror a variable duty cycle. The variable power is such that the RF signalreflected from a near target will have a lower transmitted power thanthe RF signal reflected from a more distant target at a given time. Thatis, a large amplitude RF signal is initially transmitted. Then thetransmitted power is decreased as a function of time. By the time thelarge amplitude RF signal is reflected off the more distant target andreceived, the signal strength of the reflected RF signal received fromthe near target has decreased because the transmitted power causing thereflection from the near target has also decreased. As such, the amountof interference created by the near target is reduced relative to theamount of interference that would be present for a constant powertransmitter. The signal strength of the reflected RF signal may beadapted by actually changing the amplitude of the transmitted RF signalor by changing a duty cycle of the transmitted RF signal. Thetransmission power level will start at some high level and then decreaseover time until the transmission power level reaches some minimum value,at which time the process will repeat. As an example, a target or objectwith a cross-sectional area (RCS) corresponding to the back of an18-wheeler is about 6.6 square meters (or 8.2 dBsm). At a distance of 25meters, this would produce a signal level proportional to 6.58/(25)̂4),which corresponds to a distance to the fourth power loss model (freespace path low in each direction). A correlation over a time duration of10 ms with a bandwidth of 500 MHz (which results in 5,000,000 chips)with a single receiver would produce sidelobes that are about only 33 dBlower than the peak correlation (10*½*log₁₀(5,000,000)). This is truefor PRBS code sequences because for random sequences, the sidelobes arelower than the peak by the square root of the length of the sequence.Thus, the factor of ½ outside the logarithm. On the other hand, a targetwith a much smaller radar cross section (e.g., 1 dBsm or 0 dBsm) and ata greater distance (e.g. 300 m), would produce a peak correlationproportional to 1/(2*300)⁴, which is 42 dB lower than the sidelobes ofan RF signal received from a nearer target. As a result, the moredistant target would not be detectable in the presence of the closertarget.

For example, consider a radar system attempting to detect a motorcycle(with a RCS of 0 dBsm) out to a range of 300 meters, in the presence ofa strong target (with a RCS of 40 dBsm) at a range of 25 meters, andwith a 10 ms scan (500 Mcps chip rate). The reflected RF signal off ofthe motorcycle would be approximately 83 dB smaller than the reflectedRF signal off of the strong target. The sidelobes due to the strongtarget would be only 33 dB below the peak signal level and would beabout 50 dB stronger than the peak signal of the motorcycle's reflectedRF signal. Thus, the strong target would effectively hide any smalltarget signals at a range of 300 meters. Targets with an RCS greaterthan 16 dBsm would be detectable with a 50% probability, but such wouldeffectively reduce the effective range of the radar system toapproximately 40% of the range for the smallest targets, or a range of120 meters. To alleviate this, an exemplary 50% duty cycle pulse for 4.0μs (transmitter ON for 2.0 μs, and OFF for 2.0 μs) is illustrated inFIG. 18. In an aspect of the present invention, the receiver may samplethe down-converted signal during the full cycle. From time T=0.167 μs toT (25 meters*2=50 meters->167 ns) 2.167 μs, the reflected RF signalcontains large amplitude target signals from near targets/objects. FromT=2.167 till T=−4.0 μs, the received signal contains reflected RFsignals from near targets with amplitudes that aren't large enough tointerfere with more distant targets. The power-on target is only aportion of the duty cycle, and hence reduces the link budget of thesystem by 10*log(1.833/4.000) or −3.4 dB. This will reduce the effectiverange of the radar to 82% of the range or ˜246 meters. This is asignificantly better trade off.

FIG. 13 is a graph illustrating the output of correlators with delays ofthe replica ranging from targets at 0 meters to targets at 360 meters.The assumptions made to generate the filter output include thetransmitter transmitting a continuous signal with constant power. Thereceiver correlates the received with delayed replicas of thetransmitted signal corresponding to the first 1000 chips (2 ms) of thetransmitted signal for delays that range from a target at 0 meters to atarget at 360 meters. In other words, FIG. 13 is a plot of the output ofa matched filter output which is matched to the first 1000 chips(starting at time 0). Target one is at a range of 25 meters, whiletarget two is at a range of 300 meters. Here the correlation is over acollection of 10,000 chips, at a chip rate of 500 Mcps, collected over20 μs. Target one (a near target) will have 3 dB higher amplitude thantarget two (a more distant target). The output due to the near target isillustrated in the top plot which has been normalized to have a peakoutput of 1000. Here the phase of the reflected signal is assumed to be0. The output due to the farther target (with the same normalization) isillustrated in the bottom plot. The actual output would be the sum ofthe two. These are not plotted on the same scale. The output due to themore distant target would not be visible in the scale that would bereasonable for the nearer target. The peak amplitude of the more distanttarget would not be detectable given the strength of the near target.The sidelobes of the near target have a root mean square value ofapproximately 33 (on the normalized scale), whereas the peak value ofthe far target is about 7 (on the normalized scale). Thus, the peak ofthe far target is much below the sidelobes of the near target. FIG. 14is a graph illustrating the correlator output due to the sum of the twosignals. This is what would actually be observed. Here the more distanttarget is not detectable because of the strengths of the sidelobes ofthe near target.

In order to mitigate the effect of the sidelobes from near targets, thepower level of the transmitted signal may be varied. In an aspect of thepresent invention, the power level of the transmitted RF signal may bevaried such that the transmission power decreases over time during aselected period of time and then repeats. For example, as illustrated inFIGS. 18 and 19, the transmitted power level may be adjusted between twolevels. During a first time period of 2 μs the transmitter is ON (and atfull power), then during a second time period of 2 μs the transmitter isOFF. A target at a distance of 25 meters (with a total distance of 50meters from radar transmitter to target and back to radar receiver)takes 0.167 μs. Thus, the receiver will receive this strong reflected RFsignal from time T=0.167 μs to time T=2.167 μs. On the other hand, atarget at 300 m will have a round trip time of 2.0 μs. The reflected RFsignal received after 2.0 μs will be received from time T=2.0 μs to timeT=4.0 μs (the second time period). During the first 0.167 μs of thissecond duration, the received signal may be ignored. There willtherefore be no interference from the close-in target for the time fromT=2.167 μs to time T=4 μs. However, there is a downside to thisapproach. The transmitter is only ON half the time. Furthermore, thereceiver only listens (for the more distant target) a somewhat reducedamount of time (instead of listening from 2-4 μs, the receiver listensfrom 2.167-4.0 μs). As such, these two factors give rise to a reductionin the signal level by about 3.4 dB. However, the interference from thenear in target has been completely isolated and removed.

FIG. 19 illustrates exemplary transmission and receiver processingtimelines. As illustrated in FIG. 19, the transmitter(s) may be ON for afirst time period of T=0 to T=2.0 μs, and then OFF for a second timeperiod of T=2.0 to 4.0 μs. During the second time period, the digitalreceiver may also be turned OFF to further lower digital noise coupling.If the digital receiver should need to be turned OFF to achieve thedesired level of noise, the analog signal may be saved in a buffer forlater digital processing. While the later digital processing willfurther slow the radar system and further reduce performance, it is anoption under some circumstances.

As also illustrated in FIG. 19, receiver processing may include “large”signal processing for the range bins covering 0-120 meters during thefirst time period, and then at 2.187 μs, “small” signal processing forthe range bins covering 26-300 meters may take place. Large signalprocessing includes the processing of reflected signals from neartargets while small signal processing includes the processing ofreflected signals from more distant targets. FIG. 19 illustrates thatreflected RF signals during the first time period will includeinterference from the bumper, antenna, and internal chip spillover, withthe last self-interfering target also returning during the first timeperiod. FIG. 19 also illustrates that the farthest target returnedduring the second time period will be from 300 meters. The range binaccumulations are also illustrated in FIG. 20, where the range binaccumulations during the first and second time periods are shown tooverlap for the range bins for 26-120 meters.

FIG. 15 is a graph illustrating the correlator outputs with the abovedescribed discontinuous transmission (i.e., a duty cycle where thetransmitter is turned ON for a period of time and then turned OFF for aperiod of time) with a target at 25 meters and a target at 300 meters.The top plot is the correlator outputs for the signal received from justthe first target, while the bottom plot shows the correlator outputs forjust the distant target. The output due to the near target dies outbecause the transmitted RF signal has been terminated. However, thedistant target will still cause an output for the correlators for longerdelays (range bins). Because the near target still causes an outputafter 2 μs there are still relatively large outputs due to the firsttarget. FIG. 16 is a graph illustrating the output due to the sum ofthese two targets. The first target still dominates the peak correlatoroutput due to the second target because the far target will startarriving at 2 μs while the near target will not finish arriving until2.167 μs. If the receiver ignores the output until 2.167 μs, then thesignal from the near target will be gone and the received signal willonly be from the far target. FIG. 17 shows the output of correlatorswhen the receiver only processes the signal after 2.167 μs.

The above described process of turning the transmitter power ON and thenOFF is one possible way to be able to detect a more distant target inthe presence of a near target. However, there are other ways this can bedone. An alternative is to continuously reduce transmission power levelas a function of time so that when a reflected RF signal from a moredistant target is arriving at the radar receiver, the reflected RFsignal from a near target has a significantly reduced amplitude. This isillustrated in FIGS. 21 and 22. This can be done by decreasing theamplitude of the transmitted RF signal as a function of time so that thereflected RF signal from a nearby target is actually much weaker thanthe reflected RF signal from a more distant target by the time the fartarget's reflection arrives at the radar receiver. In an aspect of thepresent invention, and as illustrated in FIG. 21, the transmitted RFsignal may be power shaped according to the equation 1/r⁴, where α isbetween 2.0 and 4.0, such that the transmitted signal power leveldecreases over time from a peak at T=0 until T=2.0 μs.

FIGS. 21 and 22 also illustrate that when a returning RF signal decayswith time according to 1/r⁴ (due to a power-shaped transmitted RFsignal), a constant signal strength is received for objects at variousdistances, such that they have a constant RCS. In other words, becauseof the constantly decreasing transmission power, the RCS of near targetswill have diminished by the time the more distance targets with similarRCS would have reflected back and finally received. So, when the signalfrom the far target is received, the signal from the near target willhave the same signal strength because the transmitted signal has beenreduced in power from the time the signal from the far target wastransmitted to the time the signal from the near target was transmitted.

Another method for reducing a reflected RF signal's power level from anear target relative to a more distant target is to turn OFF thetransmitter for some fraction of time, where the fraction of time is anincreasing function of time. For example, for the first 0.1 μs, thetransmitter is powered ON all the time, then for the second 0.1 μs, thetransmitter is powered ON 90% of the time. The fraction of time thetransmitter is powered ON continues to decrease so that by the time thereflected signal from a more distant target is arriving, the reflectedsignal from a nearby target has a duty factor that is quite small andthus creates relatively little interference. This is called pulsing.

An alternative to pulsing the transmitted RF signal to dynamicallyreduce the transmitted power level is to switch transmission frequenciesin a manner that allows the RF signal reflected off the more distantobject to be received even though a nearby target is also reflecting thetransmitted signal. In this way, the transmitter is always transmitting,but at different frequencies. For example, the radar system may transmiton frequency f₁ for 2 μs and then after switching to a differentfrequency f₂, continue transmitting for another 2 μs. A receiverattempting to detect an object/target at 300 meters distance wouldlisten on frequency f₁ during the second time interval from 2.167 μsuntil 4 μs. In this way, the RF signal received from a reflection off ofthe near target would be on a different frequency than the RF signalreflected off of the far target from time T=2.167 μs until T=4.0 μs.This method has the advantage of always transmitting (on somefrequency), but with the disadvantage of needing to be able to transmitat two different frequencies and receive at two different frequencies.The bandwidth would be divided into at least two parts in this example.

Receiver Functionality with Pulsed PMCW Radar:

In an aspect of the present invention, a radar receiver 2300,illustrated in FIG. 23, comprises an RF front end 2302, an analog todigital converter (ADC) 2304, a digital signal processor (DSP) 2306, anda memory 2308. An exemplary RF signal is transmitted by a radartransmitter from a time T=0 to T=1.832 μs, and with a chip rate of 500million chips per second. Each chip has a 2 ns duration. A transmissionduration of 1.832 μs corresponds to 916 chips being transmitted. Thedelay of a reflection from an object at a distance of 25 meters is 0.166μs (the time required for the transmitted RF signal to travel from theradar's transmitter, reflect off an object/target, and be received bythe radar's receiver). This means that there are about 83 chips roundtrip for an object that is 25 meters away. While there are 500 chipsround trip for an object that is 150 meters away, and 1000 chips roundtrip for an object that is 300 meters away. This means that thetransmitter will have been turned OFF for 168 ns before a reflected RFsignal from 300 meters is received. Additionally, the reflected RFsignal from an object 25 meters away will have already been completelyreceived by the time the reflected RF signal from the object at 300meters is received. Therefore, a receiver which turns ON at 2 μs andturns OFF at 3.832 μs will be able to process all the chips from anobject at 300 meters. However, an object that is 150 meters away willcause a reflected RF signal to be received at the receiver starting at 1μs. Since the receiver turns ON at 2 μs, the receiver will only receive416 chips of the reflected RF signal from an object at 150 meters. Assuch, the processing gain associated with nearer objects will be lessthan objects further away. Of course, the objects at a farther distancewill result in reflected RF signals that are received with less powerand will need more processing gain.

FIG. 24 illustrates a timing diagram for this scenario. Time traces areprovided for targets at 25 (Ttr1), 150 (Ttr2), and 300 (Ttr3) meters. Asillustrated in FIG. 24, the reflected RF signal (Ttr2) from the targetat 150 meters is received at 1.0 μs, while the reflected RF signal(Ttr3) from the target at 300 meters is received at 2.0 μs. As alsoillustrated in FIG. 24, the RF signal is terminated at time T=1.832 μswith the last reflected RF signal (Ttr4) from the target at 25 metersreceived at 2.0 μs. FIG. 24 also illustrates that the second reflectedRF signal (Ttr5) from the target at 150 meters is received between times1 μs and 2.832 μs, while the third reflected RF signal (Ttr6) from thetarget at 300 meters is received between times 2 μs and 3.832 μs.

In an aspect of the invention, the receiver 2300 may comprise thedigital processor 2306 illustrated in FIG. 23. The exemplary digitalprocessor 2306 determines the correlations of the received RF signals(“x”) with the required spreading codes (“α”), and as illustrated inFIG. 25, may be implemented with several FFT operations, an IFFToperation, and a mixer. The output of the ADC 2304, when the receiver2300 is ON, is used as an input to the FFT. As illustrated in FIGS. 25and 26, the output from the ADC 2304 may be padded (appended) with anequal number of zeros (e.g., 916). Meanwhile, as also illustrated inFIGS. 25 and 26, a code generator that has generated 916 chips may bezero padded (prepended).

See FIGS. 25 and 26 for exemplary FFT inputs where the ADC output ispadded (appended) and the code generator output is zero padded(prepended). An FFT is taken of each of these two zero-padded signals. Acomplex conjugate is then applied to the output of the FFT that receivesthe chips (“α”) from the code generator. The result is then mixed withthe output of the FFT that receives the received signal (“x”). Then aninverse FFT (IFFT) is taken. The result is all of the correlations atthe range bins between 25 and 300 meters. This output is then stored inmemory 2308.

In an aspect of the present invention, exemplary targets are located at25 meters, 150 meters, and 300 meters. There is an assumed propagationloss between the radar's transmitter and receiver that is proportionalto distance to the fourth power. This corresponds to a power loss ineach direction (radar transmitter to target and target to radarreceiver) proportional to the distance squared (e.g., free space pathloss). In this case, the propagation loss is 43.2 dB larger for thetarget at 300 meters when compared to the target at 25 meters. Thepropagation loss is 12.0 dB larger for the target at 300 meters whencompared to the propagation loss for the target at 150 meters. So, evenwith a receiver that ignores all received signals before time T=2.0 μs,which will ignore the reflection from the object at 25 meters, therewill be a signal from the reflection of the target at 150 meters thatwill be 12.0 dB larger (for the same RCS) than the reflected signal fromthe object at 300 meters.

Depending on the environment, an exemplary radar receiver could receivethe reflected RF signal during a shortened time window and thus reducethe window of time that the receiver processes the received RF signal.The result is that the reflected RF signals from targets that are closerto the transceiver (receiver/transmitter) than the target of interest(e.g., the one at 300 meters) will be reduced in the effect they have onthe correlations performed for a range bin at 300 meters. While thisremoves interference that is much larger than the reflected RF signalfrom the object at 300 meters, it also removes some of the desired RFsignal. For some receiver windows, the target at 300 meters becomesdetectable. This depends on the radar cross sections (RCSs) of thetargets and their relative distances.

For example, consider an exemplary target at 150 meters and anotherexemplary target at 300 meters with a 10 dB smaller RCS. The received RFsignal from the target at 300 meters will be 53.2 dB smaller than fromthe target at 150 meters. As such, the sidelobes of the correlationvalues corresponding to the target at 150 meters will be larger than themain peak of the correlation values corresponding to the target at 300meters. FIG. 27 illustrates the magnitude of the correlation values whenthe received RF signal is evaluated starting at time, T=2.0 μs andcontinuing until T=3.832 μs. Then the receiver is turned OFF, ignoringthe reflected signal until T=2.0 μs. There is no indication of thetarget at 300 meters in FIG. 27 because the RF signal reflecting fromthe target at 150 meters is dominating the RF signal reflecting from thetarget at 300 meters.

FIG. 28 illustrates normalized correlation values when the radarreceiver evaluates the received RF signal from time T=2.5 μs to T=3.832μs. The receiver ignores the received RF signal until T=2.5 μs. In FIG.28 the radar receiver has ignored a large part of the much strongerreflected RF signal from the target at 150 meters. The receiver has alsoignored some of the reflected RF signal from the target at 300 meters.However, since the RF signal reflecting from the target at 150 meters ismuch stronger than the RF signal reflecting from the target at 300meters, there is an overall improvement in recognizing the target at 300meters. FIG. 29 illustrates the normalized correlation output valueswhen the receiver ignores the received RF signal till T=2.832 μs, whichmeans the entire RF signal reflecting from the target at 150 meters isignored. Now the target at 300 meters is clearly visible.

Changes and modifications in the specifically described embodiments canbe carried out without departing from the principles of the presentinvention which is intended to be limited only by the scope of theappended claims, as interpreted according to the principles of patentlaw including the doctrine of equivalents.

1. A radar sensing system for a vehicle, the radar sensing systemcomprising: a transmitter configured for installation and use on avehicle, and configured to transmit radio signals, wherein thetransmitter is configured to turn ON and transmit during a first timeinterval of a plurality of time intervals, and configured to turn OFFand not transmit during a second time interval of the plurality of timeintervals that is after the first time interval; a receiver configuredfor installation and use on the vehicle, and configured to receive radiosignals, wherein the received radio signals include the transmittedradio signals reflected from targets in the environment; a processor,wherein the received radio signals are provided to the processor;wherein the processor processes the received radio signals during theplurality of time intervals, wherein different time intervals of theplurality of time intervals contain different signal levels of radiosignals reflected from first and second targets in the environment,wherein the first target is closer than a first threshold distance fromthe vehicle, and wherein the second target is farther than the firstthreshold distance from the vehicle; and wherein, responsive toprocessing the received radio signals during the plurality of timeintervals, the processor selects a particular time interval of theplurality of time intervals that is free of radio signals reflected offof the first target.
 2. The radar sensing system of claim 1, whereinradio signals reflected from the first target are received during thefirst time interval, and wherein radio signals reflected from the secondtarget are received during the second time interval.
 3. The radarsensing system of claim 2, wherein the radio signals reflected from thefirst target are not received during the second time interval.
 4. Theradar sensing system of claim 1, wherein the processor produces a streamof samples from the received radio signals.
 5. The radar sensing systemof claim 4, wherein the processor processes samples of radio signalsreflected off the second target that are received during the second timeinterval.
 6. The radar sensing system of claim 4, wherein the processorprocesses samples of radio signals reflected off the first target thatare received during the first time interval.
 7. The radar sensing systemof claim 1, wherein the transmitter is configured to transmit acontinuous wave radio signal.
 8. The radar sensing system of claim 1,wherein, during the first time interval, the received radio signalscomprise radio signals reflected from the first target, and wherein,during the second time interval, the received radio signals compriseradio signals reflected from the second target that are smaller than theradio signals reflected from the first target during the first timeinterval.
 9. The radar sensing system of claim 8, wherein radio signalsreflected from the first target during the second time interval are notlarge enough to interfere with the radio signals reflected from thesecond target.
 10. The radar sensing system of claim 1, wherein, duringthe first time interval of the plurality of time intervals, radiosignals reflected from the first target are processed for range bins fora first distance range, and wherein, during the second time interval ofthe plurality of time intervals, radio signals reflected from the secondtarget are processed for range bins for a second distance range, andwherein at least a portion of the second distance range is farther waythan the first distance range.
 11. The radar sensing system of claim 1,wherein the processor is configured to select a time interval of theplurality of time intervals to process received radio signals for aselected plurality of range bins.
 12. The radar sensing system of claim11, wherein the processor processes received radio signals for rangebins associated with a range of 0-120 meters during the first timeinterval, and wherein the processor processes received radio signals forrange bins associated with a range of 26-300 meters during the secondtime interval.
 13. The radar sensing system of claim 1, wherein, duringthe first time interval, the receiver is turned OFF and the reflectedradio signals are ignored, and wherein, during the second time interval,the receiver is turned ON and the reflected radio signals are received.14. The radar sensing system of claim 1, wherein, during the first timeinterval, digital processing is turned OFF and received radio signalsare saved to a buffer for later processing.
 15. The radar sensing systemof claim 1, wherein the transmitter is configured to power shape thetransmitted radio signals during the first time interval.
 16. The radarsensing system of claim 15, wherein, during the first time interval, thetransmitted radio signals are decreasing in power.
 17. The radar sensingsystem of claim 15, wherein the transmitter is configured to pulse thetransmitted radio signals with a plurality of duty cycles during thefirst time interval.
 18. The radar sensing system of claim 1, whereinthe first target comprises a plurality of first targets, and whereineach of the plurality of first targets is closer than the firstthreshold distance from the vehicle, and wherein the second targetcomprises a plurality of second targets, and wherein each of theplurality of second targets is farther than the first threshold distancefrom the vehicle.
 19. A radar sensing system for a vehicle, the radarsensing system comprising: a transmitter configured for installation anduse on a vehicle, and configured to transmit radio signals, wherein thetransmitter is configured to pulse the transmitted radio signals with aduty cycle that reduces over time; a receiver configured forinstallation and use on the vehicle, and configured to receive radiosignals, wherein the received radio signals include the transmittedradio signals reflected from targets in the environment, wherein thetargets in the environment comprise first and second targets, whereinthe first targets are closer than a first threshold distance from thevehicle, and wherein the second targets are farther than the firstthreshold distance from the vehicle; wherein the received radio signalsare provided to a processor; wherein the processor processes thereceived radio signals during a plurality of time intervals, and whereindifferent time intervals of the plurality of time intervals containdifferent signal levels of radio signals reflected from the targets inthe environment; and wherein, responsive to processing the receivedradio signals during the plurality of time intervals, the processorselects a particular time interval of the plurality of time intervalsthat is free of radio signals reflected off of the first targets. 20.The radar sensing system of claim 19, wherein pulsing the transmittedradio signals with a duty cycle that reduces over time decreases a powerlevel of the transmitted radio signals over time.
 21. The radar sensingsystem of claim 20, wherein the processor processes the received radiosignals that are reflected from the first targets during a first timeinterval of the plurality of time intervals, and wherein the processorprocesses the received radio signals that are reflected from the secondtargets during a second time interval of the plurality of time intervalsthat is after the first time interval.
 22. The radar sensing system ofclaim 21, wherein, during the first time interval, the received radiosignals comprise radio signals reflected from the first targets, andwherein, during the second time interval, the received radio signalscomprise radio signals reflected from the second targets that aresmaller than the radio signals reflected from the first targets duringthe first time interval, and wherein radio signals reflected from thefirst targets during the second time interval are not large enough tointerfere with the radio signals reflected from the second targets. 23.A radar sensing system for a vehicle, the radar sensing systemcomprising: a transmitter configured for installation and use on avehicle, and configured to transmit radio signals; a receiver configuredfor installation and use on the vehicle, and configured to receive radiosignals, wherein the received radio signals include the transmittedradio signals reflected from targets in the environment, and wherein thetargets in the environment comprise first and second targets, andwherein the first targets are closer than a first threshold distancefrom the vehicle, and wherein the second targets are farther than thefirst threshold distance from the vehicle; wherein the receiver receivesradio signals during selected time intervals of a plurality of timeintervals, and wherein a particular time interval of the plurality oftime intervals is free of radio signals reflected from the firsttargets; wherein the received radio signals are provided to a processor;wherein the processor processes the received radio signals during theplurality of time intervals, and wherein different time intervals of theplurality of time intervals contain different signal levels of radiosignals reflected from the targets in the environment.
 24. The radarsensing system of claim 23, wherein, during a first time interval of theplurality of time intervals, radio signals reflected from the firsttargets are processed for range bins for a first distance range, andwherein, during a second time interval of the plurality of timeintervals that is after the first time interval, radio signals reflectedfrom the second targets are processed for range bins for a seconddistance range, and wherein at least a portion of the second distancerange is farther way than the first distance range.
 25. The radarsensing system of claim 24, wherein, during the first time interval, thereceived radio signals comprise radio signals reflected from the firsttargets, and wherein, during the second time interval, the receivedradio signals comprise radio signals reflected from the second targetsthat are smaller than the radio signals reflected from the first targetsduring the first time interval, and wherein radio signals reflected fromthe first targets during the second time interval are not large enoughto interfere with the radio signals reflected from the second targets.